Absolutely right, A is colloquial shorthand and not grammatically proper. Adding who improves the text but doesn't fix it; there's still an unsemantic tense use. How can you close the curtains if you've already left the office? Both B and C are acceptable and correct remedies.
B indicates you should, as part of the process of leaving, close the curtains.
C specifies that the curtains are to be closed immediately before exiting. The use of present tense tells us that the two actions happen at (effectively) the same time, and logic tells us that the curtains must be closed before you've left. Whether or not closing the curtains is part of the process of leaving is technically ambiguous but irrelevant.
(If you are the last person to leave applies in both cases, obviously.)
You're also spot on with the addition of the; it should indeed read the curtains. Technically speaking, without the it's unspecified which curtains should be closed, though the meaning is readily evident without the article.
Bonus alternative: you might also fix the sentence like this:
Please close the curtains if you are the last one left in the office.
Here, one need not be leaving to be obligated to close the curtains.
It's you who has chosen.
It's you who have chosen.
The former isn't grammatical whereas the latter is OK.
It's a cleft sentence divided into two clauses. The first clause "it's you" is a focussed clause. The verb in the second clause agrees with the subject in the first clause; if the subject is in the singular or plural, the verb in the other clause is used accordingly. But, as the pronoun you, whether in the singular or plural, always takes a plural verb, you must use have, not has, in the sentence presented.
Cambridge Dictionary online
Best Answer
Though normally a human antecedent is followed by who, yet in some cases that is also accepted. Both of these sentences quoted above are correct.
From The New Fowler's Modern English Usage - R. W. Burchfield page no. 773